Air charge estimation for use in engine control

ABSTRACT

Methods, devices, estimators, controllers and algorithms are described for estimating working chamber air charge during engine operations. The described approaches and devices are well suited for use in dynamic firing level modulation controlled engines. Manifold pressure is estimated for a time corresponding to an induction event associated with a selected working cycle. The manifold pressure estimate accounts for impacts from one or more intervening potential induction events that will occur between the time that the manifold pressure is estimated and the time that the induction event associated with the selected working cycle occurs. The estimated manifold pressure is used in the estimation of the air charge for the selected working cycle. The described approach may be used to individually calculate the air charge for each induction event at any time that the engine is operating in a mode that can benefit from the individual cylinder air charge estimations.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority of U.S. Provisional PatentApplication Nos. 62/353,218, filed on Jun. 22, 2016, and 62/362,177,filed on Jul. 14, 2016, each of which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to estimating air charge invehicles powered by internal combustion engines.

BACKGROUND

Fuel efficiency of internal combustion engines can be substantiallyimproved by varying the displacement of the engine in response to thedemanded torque. Full displacement allows for the full torque to beavailable when required, yet using a smaller displacement when fulltorque is not required can significantly reduce pumping losses andimprove thermal efficiency. The most common method today of implementinga variable displacement engine is to deactivate a group of cylinderssubstantially simultaneously. In this approach the intake and exhaustvalves associated with the deactivated cylinders are kept closed and nofuel is injected when it is desired to skip a combustion event. Forexample, an 8-cylinder variable displacement engine may deactivate halfof the cylinders (i.e. 4 cylinders) so that it is operating using onlythe remaining 4 cylinders. Commercially available variable displacementengines available today typically support only two or at most threedisplacements.

Another engine control approach that varies the effective displacementof an engine is referred to as “skip fire” engine control. In general,skip fire engine control contemplates selectively skipping the firing ofcertain cylinders during selected firing opportunities. Thus, aparticular cylinder may be fired during one engine cycle and then may beskipped during the next engine cycle and then selectively skipped orfired during the next. Skip fire engine operation is distinguished fromconventional variable displacement engine control in which a designatedset of cylinders are deactivated substantially simultaneously and remaindeactivated as long as the engine remains in the same variabledisplacement mode. Thus, the sequence of specific cylinders' firingswill always be the same for each engine cycle during operation in avariable displacement mode (so long as the engine remains in the samedisplacement mode), whereas that is often not the case during skip fireoperation. For example, an 8-cylinder variable displacement engine maydeactivate half of the cylinders (i.e. 4 cylinders) so that it isoperating using only the remaining 4 cylinders.

In general, skip fire engine operation facilitates finer control of theeffective engine displacement than is possible using a conventionalvariable displacement approach. For example, firing every third cylinderin a 4-cylinder engine would provide an effective displacement of ⅓^(rd)of the full engine displacement, which is a fractional displacement thatis not obtainable by simply deactivating a set of cylinders.Conceptually, virtually any effective displacement can be obtained usingskip fire control, although in practice most implementations restrictoperation to a set of available firing fractions, sequences or patterns.The Applicant has filed a number of patents describing variousapproaches to skip fire control. By way of example, U.S. Pat. Nos.8,099,224; 8,464,690; 8,651,091; 8,839,766; 8,869,773; 9,020,735;9,086,020; 9,120,478; 9,175,613; 9,200,575; 9,200,587; 9,291,106;9,399,964, and others describe a variety of engine controllers that makeit practical to operate a wide variety of internal combustion engines ina dynamic skip fire operational mode. Each of these patents isincorporated herein by reference. Many of these patents relate todynamic skip fire control in which firing decisions regarding whether toskip or fire a particular cylinder during a particular working cycle aremade in real time—often just briefly before the working cycle begins andoften on an individual cylinder firing opportunity by firing opportunitybasis.

In some applications referred to as dynamic multi-level skip fire,individual working cycles that are fired may be purposely operated atdifferent cylinder outputs levels—that is, using purposefully differentair charge and corresponding fueling levels. By way of example, U.S.Pat. No. 9,399,964 describes some such approaches. The individualcylinder control concepts used in dynamic skip fire can also be appliedto dynamic multi-charge level engine operation in which all cylindersare fired, but individual working cycles are purposely operated atdifferent cylinder output levels. Dynamic skip fire and dynamicmulti-charge level engine operation may collectively be considereddifferent types of dynamic firing level modulation engine operation inwhich the output of each working cycle (e.g., skip/fire, high/low,skip/high/low, etc.) is dynamically determined during operation of theengine, typically on an individual cylinder working cycle by workingcycle (firing opportunity by firing opportunity) basis. It should beappreciated that dynamic firing level engine operation is different thanconventional variable displacement in which when the engine enters areduced displacement operational state, a defined set of cylinders areoperated in generally the same manner until the engine transitions to adifferent operational state.

The fuel control system in many engine controllers includes an aircharge estimator, such as a MAC (mass air charge) estimator or an APC(air per cylinder) estimator. Conventional air estimation techniques areoften not particularly well suited for use in skip fire controlledengines due to the impacts of irregular and/or shifting firing sequencesthat can occur during skip fire operation. Many prior art air chargeestimators utilize a mean or a filtered value of a measured absolutemanifold pressure (MAP) in their air charge estimations. Often, use ofmean pressures will not accurately predict the cylinder air charge on afiring-by-firing basis because previous MAP values are not necessarilyindicative of the future trajectory of the manifold pressure in a skipfiring engine. In addition, changes in the position of other airflowcontrol actuators such as throttle and cam phasers taking place afterthe cylinder air estimate is required for fuel injection amountdetermination are not accounted for. Fuel injection based on poorlypredicted air charges will result in a mixture of rich and leancombustions, causing poor catalytic converter efficiency, and whenextreme, loss in torque and efficiency as combustion of the excess fuelin rich-firing cylinders will be completed inside the catalyticconverter.

U.S. patent application Ser. No. 13/794,157, U.S. Provisional PatentApplication No. 62/068,391 and SAE Technical Paper 2015-01-1717 eachdescribe air charge estimation techniques suitable for use in skip fireengine operation. Each of these references is incorporated herein byreference. Although such estimators work well in many applications,there are continuing efforts to further improve the air chargeestimators and estimation techniques in manners that are well suited foruse in skip fire and multi-level engine control schemes.

SUMMARY

To achieve the foregoing and other objects of the invention, methods,devices, estimators, controllers and algorithms are described forestimating working chamber air charge during engine operations. Thedescribed approaches and devices are well suited for use in dynamicfiring level modulation controlled engines as well as for use in otherskip fire and multi-charge level operating modes. In some embodiments,manifold pressure is estimated for a time corresponding to an inductionevent associated with a selected working cycle. The manifold pressureestimate is arranged to account for impacts from one or more interveningpotential induction events that will occur between the time that themanifold pressure is estimated and the time that the induction eventassociated with the selected working cycle occurs. The estimatedmanifold pressure may then be used in the estimation of the air chargefor the selected working cycle. Such an approach may be used toindividually calculate the air charge for each induction event at anytime that the engine is operating in a mode that can benefit from theindividual cylinder air charge estimations.

The estimated air charge may be used by a fuel charge estimator or othersuitable device in the determination of a desired fuel charge for theselected working cycle. The desired fuel charge can then be injected bya fuel injection controller at the appropriate time for the selectedworking cycle.

The air charge estimate is preferably made as late as possible beforethe fueling pulse occurs, to accurately account for changing engineoperating conditions resulting from dynamically changing torque requestsfrom the engine, so that a target combustion air-fuel ratio is achieved.This late-as-possible time is usually after the corresponding firingdecision for the cylinder has been made, but before the correspondinginduction event is complete. The firing decision is preferably made atclose to the time that the corresponding working cycle begins, but itmust be made far enough before the working cycle begins so that there isadequate time to activate and/or deactivate any actuators that arerequired to deliver (or deny) the air charge required to implement thefiring decision (which can take the form of deactivating a correspondingworking chamber in the case that the firing decision is to skip theselected working cycle).

In some embodiments, air charge estimation utilizes a then currentfiring history of the working chamber in which the selected workingcycle occurs in the determination the air charge. Similarly, a manifoldpressure estimator or other suitable device can utilize the then currentfiring histories of any other working chambers utilized for interveninginduction events in the manifold pressure estimation.

Is some embodiments the state of various actuators and/or parametersthat are relevant to the manifold pressure and/or air chargedetermination expected at the time of the induction event associatedwith the selected working cycle may be predicted by appropriatepredictors and utilized in the manifold pressure and/or air chargeestimations. These may include predicting values for parameters such ascam phase, engine speed, manifold temperature, boost level, throttleposition, air inlet pressure and temperature, exhaust pressure, etc.

In some embodiments, the estimated manifold pressure corresponds to amanifold pressure that is expected just before or coincident with theclosing an intake valve actuated to facilitate the induction eventassociated with the selected working cycle. By way of example, theestimated manifold pressure may correspond to an expected pressure inthe range of zero to forty crank angle degrees before the intake valveclosure.

In some embodiments, an initial air charge estimation is made in therange of approximately 180° to 1080° of crank angle rotation prior tothe opening of the intake valve actuated to facilitate the inductionevent associated with the selected working cycle. The initial air chargeestimation may be refined by using updated information on actuatorposition and MAP as time between the estimation and the induction eventdecreases. A final air charge estimation may be determined at or nearthe end of the induction event. Depending on the required fuel injectiontiming either the initial air charge estimate, the final air chargeestimate, or some intermediate air charge estimate may be used to helpdetermine the injected fuel mass. This final air charge estimation maybe used in conjunction with the injected fuel mass and measurements ofexhaust constituents to improve the accuracy of the air chargeestimation.

In some embodiments, the air charge estimation is further based in parton whether an exhaust valve that vents the working chamber in which theselected working cycle occurs was actuated in the immediately precedingworking cycle in that working chamber. In general, the estimated aircharge will be higher when the exhaust valve that vents the workingchamber in which the selected working cycle occurs was not actuated inthe immediately preceding working cycle in that working chamber relativeto what the estimated air charge would have been if, all other factorsbeing equal, the exhaust valve had been actuated in the immediatelypreceding working cycle.

In some embodiments, estimates of the inflow of air into the intakemanifold together with estimates of the outflow of air associated witheach induction event are used in the manifold pressure estimation.

In some embodiments, the estimated air charge is determined using aspeed density calculation.

The described estimators, controllers, devices, methods and algorithmsmay be used in conjunction with a wide variety of different engines. Theadvantages of using the described approach tend to increase as the ratiobetween intake manifold volume and individual working chamberdisplacement gets smaller. Low cylinder count engines, such as 2, 3, and4 cylinder engines, tend to have a smaller intake manifold volume toindividual working chamber displacement ratio than high cylinder countengines, such as 6 or 8 cylinder engines. Thus, the present invention isparticularly applicable to 2, 3, and 4 cylinder engines. For example,the ratio of intake manifold volume to individual cylinder displacementmay be in the range of 15 to 20 for an 8-cylinder engine, while for a4-cylinder engine it may be less than 10.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and the advantages thereof, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings in which:

FIG. 1 shows MAP variation with all-cylinder firing.

FIG. 2 shows MAP variation with a fire-fire-skip (repeating) firingsequence.

FIG. 3 shows an exemplary timing diagram illustrating timing of firingdecisions, valve actuations, and fuel injection.

FIG. 4 is a block diagram illustrating an air charge estimator thatincludes a manifold model with time alignment of firing decision inputsaccording to an embodiment of the present invention.

FIG. 5 shows an air charge estimator block diagram including predictionof several inputs according to an embodiment of the present invention.

FIG. 6 is a flow chart illustrating a method of determining MAC in askip-fire controlled engine according to an embodiment of the presentinvention.

FIG. 7 is a diagram of an air charge estimator in accordance withanother embodiment.

FIG. 8 is a diagram of another air charge estimator suitable for use inconjunction with multi-level skip fire operation.

FIG. 9 is a flow chart illustrating a method of providing successivelyimproved estimations of MAC in a skip-fire controlled engine accordingto an embodiment of the present invention.

In the drawings, like reference numerals are sometimes used to designatelike structural elements. It should also be appreciated that thedepictions in the figures are diagrammatic and not to scale.

DETAILED DESCRIPTION

The present invention relates to methods and systems for improving theaccuracy of the air charge estimations used in the control of aninternal combustion engine. The described techniques are particularlywell suited for use in conjunction with skip-fire and multi-charge levelengine control, although they may readily be used in other circumstancesas well.

In dynamic skip fire (DSF) and other dynamic firing level modulationengine controllers, firing decisions need to be made in advance of theimplementation of the decision. This allows the engine control module(ECM), power train control module (PCM), engine control unit (ECU) orother appropriate engine controller(s) time to prepare various actuatorsto be placed in their proper condition to implement thefire/no-fire/firing-level decision. The relevant actuators include thecylinder activation/deactivation system, fuel injectors, the ignitioncoils, etc. required to implement the firing decision. In variousdescribed embodiments, an air charge estimator is configured to predictor look ahead in time in order to properly estimate the air chargeinducted into a cylinder. Better air charge estimates facilitate moreaccurate fuel charge injection to meet a target air-fuel ratio, whichhas the potential to improve both fuel efficiency and emissioncharacteristics.

The described approaches are particularly well suited for use in skipfire and other firing level modulation controlled engines. In someembodiments, the firing decisions, along with a model of the intakemanifold, are used to provide a more accurate air charge estimate foreach future firing cylinder. Better overall air-fuel ratio regulationand in particular better firing-by-firing air-fuel ratio uniformity maybe obtained. Improved air-fuel regulation improves the efficiency of athree-way catalytic converter that may be located in the engine exhaustsystem, resulting in better regulatory conformance of tailpipeemissions, or alternatively, better cost-optimization of the catalyticconverter.

An air charge estimator can use the known values of intake manifoldpressure and temperature, cam phase, throttle position and other enginevariables at the time the fire/no-fire/fire-level decision is made. Itmay also use any anticipated changes in actuators such as throttle orcam in the time between the firing decision and intake valve closure(IVC) of the firing cylinder. The air charge estimator also knowswhether any intervening firing opportunities will be a skip or a fire.The air charge estimator may then use this information to predict theMAP at a time slightly prior to or coincident with IVC of a firingcylinder. The air charge estimator may provide successively moreaccurate predictions of the air charge, starting with an initialestimate at the time of the firing decision to a final estimate at ornear IVC.

FIG. 1 shows the manifold absolute pressure (MAP) variations occurringin an exemplary engine running at steady-state with all cylindersfiring. In FIG. 1 the solid curve is the measured MAP and the dashedcurve is an estimated MAP derived from a model of intake manifolddynamics. The average MAP is approximately 40 kPa and the variation inthe MAP is less than 4 kPa. Maximum MAP occurs at intake valve opening(IVO) as a cylinder starts its working cycle. Minimum MAP occurs at ornear intake valve closure (IVC). In all-cylinder operation, the valuesof MAP at IVO and IVC are relatively constant from firing to firing, soregardless of the angular location of the air estimation algorithmexecution, the algorithm can be calibrated properly, and the MACestimate associated with each successive cylinder firing can becalculated relatively accurately based on previous MAP information.

The situation is different for a skip-fired controlled engine where notevery working cycle results in a cylinder firing—and particularly whenfiring sequences are used that do not always have the same number ofskips that occur before every firing. FIG. 2 shows the MAP versus timefor an engine operating at steady state in a fire-fire-skip pattern,which corresponds to firing fraction of ⅔ with most evenly spacedfirings. As in FIG. 1 the solid line is measured MAP and the dashed lineis modeled MAP. The situation here is very different than the allcylinder firing case depicted in FIG. 1. The manifold pressure varies byabout 10 kPa, more than twice the variation observed in the all cylinderfiring case. Also, the MAP at or near IVC varies significantly betweensuccessive engine cycles. The difference in the measured MAP betweenIVC₁ and IVC₂ is about 3 kPa, approximately 3× larger than the variationseen in all cylinder operation. The most important time windowinfluencing cylinder MAC is a time window around closing of the intakevalve (IVC) as manifold conditions around this time largely determinethe amount of manifold gas inducted into a cylinder. Before this timechanging gas pressures are equalizing between intake manifold andcylinder, and after this time the intake valve is closed and manifoldconditions become no longer relevant to the trapped air mass in thecylinder.

Dynamic skip fire operation most commonly includes cylinder deactivationwhereby intake and exhaust valves are kept closed during the nominal gasexchange phases of the 4-stroke engine cycle. Performing cylinderdeactivation requires the engine controller to control power driveroutputs that actuate the cylinder deactivation elements. For camoperated valves, cylinder deactivation may be realized by actuatingsolenoids that operate hydraulic oil control valves, which allow thevalve lifters to either remain rigid (a fired cylinder) or collapsed (askipped cylinder). Such a system may be referred to as a “lost-motion”deactivation system. Cylinder deactivation can also be achieved usingmechanisms other than cams. For example, electromechanical actuators(e.g. solenoids) may be used to control the intake and/or the exhaustvalves.

As described in U.S. patent application Ser. No. 14/812,370, which isincorporated herein by reference, there is a time lag between afire/no-fire decision and implementation of that decision. Depending onthe mechanism used to deactivate the cylinders and the number ofcylinders in the engine, this delay can be in the range of 2 to 9 firingopportunities. This delay can be expressed in terms of degrees of crankangle and may, for example, be between 180° and 1080° of crankshaftrotation depending on the control algorithm and deactivation systemused. Therefore, if air charge is calculated in the engine controllerusing inputs measured at the time of the firing decision, the air chargecalculation may be inaccurate.

The time of decision for valve actuation is typically in advance of thetime that an air estimate is needed for purposes of calculating correctfuel injection parameters to achieve a desired cylinder air-to-fuelratio. Based on intake valve actuation of other cylinders in the engine,the intake manifold pressure may decrease or increase from its value atthe time of the firing decision. These intake manifold pressure changecan be modeled and that model used to help predict the air charge. Morespecifically, at the time that any particular firing decision is made,the earlier firing decisions for any other cylinders that canpotentially have induction events between the particular firing decisionand its associated induction event will be known. Thus, regardless ofthe time that corresponding air charge is determined, the model canaccount for any intervening induction events that will (or won't) occurbefore the induction event corresponding to that particular firingdecision is made. Considering the impacts of such intervening inductionevents allows the future trajectory of the intake manifold pressure tobe more accurately predicted, allowing more accurate prediction of thecylinder inducted air mass. More accurate air charge estimation resultsin a cylinder air-fuel ratio more closely matching the desired air-fuelratio, minimizing variations in the chemical species products ofcombustion. This improves fuel efficiency and has the potential tofacilitate the use of smaller catalytic converters with correspondingcost savings.

FIG. 3 illustrates the relative timing of various control and actuatorevents that can occur in a dynamic skip fire (DSF) engine controlsystem. Shown here are exemplary pulses for fuel injection, liftprofiles for the intake and exhaust valves, and timing of the firingdecision. This figure depicts one full engine cycle, denoted as the(n−1)^(th) cycle and portions of the (n−2)^(th) and n^(th) cycle. Then^(th) cycle starts at a top dead center (TDC) crank orientation andcorresponds to 0° of crank angle. The decision on whether to fire on then^(th) cycle must be made prior to the start of the n^(th) cycle. Inthis example, that decision is made at approximately 785° of crank angleprior to the start of the n^(th) cycle. This lead time is required toimplement the firing decision. For example, in the case of cam operatedvalves with a “lost-motion” valve lifter, the hydraulicactivation/deactivation system needs time to open an oil control valveand to build hydraulic pressure to move a spring-loaded mechanical lockinto position, which then allows the lifter to transfer mechanicalmotion of the cam to the intake valve. Fueling will also changedepending on whether the n^(th) cycle is a skip or fire. Since in thiscase a decision to fire has been made, the cylinder needs to be fueled.The fueling subsystem is given the command to fuel and the fuel pulsetiming parameters at approximately 405° of crank angle prior to thestart of the n^(th) cycle. The actual fueling pulse extends fromapproximately −315° to +45° of crank angle relative to the start of then^(th) cycle. Note that in the illustrated embodiment, the fuelingparameters for the n^(th) cycle need to be defined prior to the start ofthe n^(th) cycle and the n^(th) cycle intake valve opening.

Also labeled in FIG. 3 are IVO and IVC locations for the n^(th) cycle.These occur at approximately −45° and 315° of crank angle for IVO andIVC, respectively. The time window where the MAP most closely correlatedwith the n^(th) cycle MAC occurs during the period in which the intakevalve is open. That is, between IVO and IVC. More specifically, in theillustrated embodiment, the MAP that is most relevant to the MACcalculation tends to be in the latter portion of the intake valveopening, particularly in a timing window 0° to 40° prior to IVC, around290° of crank angle in this case. It should be appreciated that thevalues given in FIG. 3 are exemplary only and they can changesignificantly based on factors such as the engine architecture andcurrent operating conditions. In particular, the fuel injection windowshown is appropriate for a port fuel injected engine. Direct fuelinjected engines would have a later injection window.

Once a decision to fire has been made it is necessary to estimate theMAC associated with the firing. Accurate knowledge of the MAC isimportant to control the fuel/air ratio, which is generally held at ornear a stoichiometric ratio for engines with conventional 3-waycatalyst, or an optimum lean value for engines with a leanaftertreatment system. Once the MAC is estimated the mass of injectedfuel can be determined to yield the desired combustion stoichiometry.

FIG. 4 is a schematic view of a model-based MAC estimation system 400according to an embodiment of the present invention. The MAC estimatormay use an intake manifold inflow model 410 having as inputs throttleposition 412 along with various measured or estimated signals from inletand manifold sensors or estimators, such as inlet air pressure 414 andinlet air temperature 415. In some embodiments, additional inputs 416may be used such so that intake manifold inflows from an exhaust gasrecirculation system, exhaust gas backflow (intake valve venting exhaustgases into the intake manifold), or boost levels from a turbocharger orsupercharger may be calculated. In other embodiments inputs from a massairflow (MAF) sensor (not shown) may be used in addition to or in placeof one or more of the other inputs.

The MAC estimator 400 may also include a manifold outflow model 430which includes inputs such as firing decisions 432, cam phases 434,engine speed 436, intake manifold temperature 437 and other variables438 as appropriate for the model used. Other variables may include thefiring history of the cylinder in question, since this influences thecylinder air charge as described in U.S. patent application Ser. No.13/843,567, which is incorporated herein by reference. Key to theprediction of MAC is the fact that firing decisions are known inadvance, both for the cylinder in question and for all cylinders thathave a firing opportunity between the firing decision point and IVC ofthe cylinder in question. Thus, irregular intake manifold outflowscaused by skip fire operation can be modeled.

Outputs of the intake manifold inflow estimator 410 and intake manifoldoutflow estimator 430 can be input into an intake manifold pressuremodel 450, which is arranged to predict the manifold pressure on anindividual cylinder working cycle basis. That is, for each cylinderworking cycle/firing opportunity, the intake manifold pressure modelpredicts the manifold pressure a time in the future corresponding tothat working cycle's potential intake event (potential intake eventbecause if the working cycle is skipped, the cylinder may not have anassociated intake event during that working cycle). The output of themanifold pressure model is the estimated manifold pressure 455 which maybe fed back to both the inflow estimator 410 and outflow estimator 430.The estimated manifold pressure 455 is also input into a MAC estimator470 which is configured to determine the mass air charge (MAC) 475 foreach induction event on an individual cylinder by individual cylinderbasis.

Other inputs into the MAC estimator 470 may include the firing decision432, the cam phase angle(s) 434, and other inputs 438 relevant to theMAC calculation. The intake cam phase angle determines IVO and IVCtiming and thus the appropriate time window for the MAP value to be usedin a MAC calculation. The exhaust cam phase angle determines possibleoverlap in intake/exhaust valve opening, which may influence freshcharge flow into the cylinder. Any suitable MAC estimator may be used.For example, MAC estimator 470 may utilize a conventional speed densityMAC calculation. By way of example, one suitable speed densitycalculation utilizes engine speed, intake port temperature, cam phase,and exhaust pressure as its input variables. The output of the MACestimator 470 is an estimated MAC value 475, which may be input into thefueling algorithm of an ECU, power train controller or other type ofengine controller.

In the embodiment described above, the future air charge is predicted ina model based manner using an intake system model with not only theconventional predicted inputs, but the already-decided future individualintake valve decisions so that a time aligned prediction is achieved.Since in the case of the timing diagram of FIG. 3 the firing decision ismade at −785° of crank angle, the future state of the intake valve ofthis cylinder is known, or can be reasonably estimated if the cam phaseis changing, at the time the firing decision is made. Thus thisinformation, as well as that of cylinders prior in the firing orderwhose intake events will occur between −785° and 0° of crank angle, isavailable to predict the future trajectory of the MAC influencingvariables in advance of the time that MAC estimation occurs.

Some of the inputs to the MAC estimator can be predicted based on thecurrent behavior of an actuator. For example, a throttle or cam phaserpossesses mass and according to Newton's first axiom of motion an objectin motion tends to stay in motion. Therefore the current and pasttrajectory of the actuator (e.g. throttle position vs time) can be usedto predict its future trajectory.

In other cases, the reference inputs to the closed loop controllers forsubsystems are also known in advance, such as target cam phases, targetthrottle position, target waste gate positions or target turbochargervariable geometry turbine positions. Knowing the dynamics of the closedloop systems, such as the controller algorithm and plant model, temporalchanges in the model inputs can be predicted.

FIG. 5 indicates an alternative embodiment of a MAC estimator. This MACestimator 500 is similar to that shown in FIG. 4 and its overalldescription will not be repeated. In FIG. 5, unlike FIG. 4, predictionof the future state of inputs is explicitly incorporated. Thusanticipated or estimated changes in any of the input variables caninfluence the output MAC. It should be appreciated that both FIG. 4 andFIG. 5 depict possible embodiments of a control system. A control systemwith similar functionality may be implemented in different ways or maycombine or separate the various functions in a different manner. Itshould also be appreciated that not all inputs may be required in allcases and additional inputs may be necessary depending on the enginearchitecture. It should also be appreciated that not all inputs to themodel need to be predicted. Thus, in many instances using then-currentmeasurements for some or all of the various model inputs (other thaninputs directly based on the firing decisions) provides significantlybetter estimates than traditional MAC modeling approaches.

Once the MAC for the firing cylinder is determined, the appropriateinjected fuel mass can be readily calculated based on the desiredcombustion air-fuel ratio. The required prediction horizon for fuelinjection is dependent on the control software architecture as well asfuel injector characteristics. Fuel injection may require many crankangle degrees to execute, particularly for port fuel injection at highengine speed. Even for direct injection, fueling may be started prior toIVC. The closing of the working chamber intake valve will occur manycrank angle degrees later than when an estimate of air charge is neededin order to program the fuel injector driver. Timing and duration of thefuel injection pulse is typically implemented with a dedicated timingperipheral integrated into the microcontroller unit of the ECM such as aTPU (time processor unit). If the MAC is not accurately determined theamount of injected fuel may be incorrect for the desired cylinderair-fuel ratio.

While predictions can be made regarding the actuator positions andengine parameters, there is inevitably some discrepancy between thesepredictions and the actual positions and parameters. In someembodiments, the MAC estimator 400 or 500 may be utilized one or moretimes between making an initial MAC estimate at or near the time of thefiring decision and a final MAC estimate at or near IVC. The knownfiring pattern and updated values of actuator positions and engineparameters may be input into the MAC estimator 400 to obtain asuccessively more accurate MAC estimation.

FIG. 6 shows a method 600 of estimating MAC in a skip-fire controlledengine according to an embodiment of the current invention. The methoduses an intake manifold flow and pressure model as part of a predictivescheme. If the inputs to the model of the manifold are predicted forwardin time, then the MAC output of the model will be predictive and will beaccurate to the extent the predicted inputs to the model match theactual trajectory of those inputs.

The method starts at step 610 with a decision to fire a cylinder in askip-fire controlled engine. If the decision is to not fire, i.e. skip,then the flow method may move directly to the next firing decision,where the method 600 is repeated. If the decision is made to fire, theMAP at the time of the firing decision or soon thereafter is determinedin step 620. The determination can be either by a direct measurement, aMAP model, or some combination of the two methods. The MAP determinationneeds to be made prior to fuel injection for the firing cylinder. A timewindow for obtaining a MAP estimate is then determined at step 640 basedon the opening and/or closing of the firing cylinder's intake valve. Thetime window is preferably during the last half of the intake valvesopening and in particular in a time window 0° to 40° of crank angleprior to intake valve closing.

In step 630 a model predictive of MAP behavior is used to estimate MAPduring the time window. In particular, the model includes the impact ofany induction events, or skipped induction events, by other cylindersthat occur prior to the closing of the intake valve on the firingcylinder. The model may further utilize sensor measurements of thecurrent values of various engine parameters and/or actuators. The modelmay further estimate how these values may change between the initial MAPdetermination and the MAP estimation during the time window. Changes inthese values may be used to further refine the MAP estimation. Forexample, the model may take into account feed forward control on thethrottle position to help control the MAP. The model may also includethe firing history of the cylinder. The model may further include thefiring history of other engine cylinders.

At step 650 the MAC is estimated for the firing cylinder using theestimated MAP in the relevant time window determined in step 640. TheMAC estimation must be made prior to determination of the fuelingparameters, which occur at step 656. The injected fuel mass may bechosen and programmed into a fuel injection system to provide theappropriate combustion stoichiometry. At step 660 the fuel injectionsystem injects the determined air fuel mass. At step 670 the air/fuelcharge is combusted in the fired cylinder. The cycle is then repeatedfor the next firing opportunity. Each cylinder in the engine capable ofskip-fire control may use a control method similar to that shown in FIG.6. The firing decisions and implementation of those firing decisions foreach cylinder are temporally interleaved. That is a decision on whetherto skip or fire is made at every firing opportunity. If the decision isto fire, the method 600 or some similar method may be used to determinethe MAC and injected fuel mass for that firing. While the method 600 isbeing implemented on one cylinder, the method may be performed for theengine's other cylinders with a temporal offset.

Referring next to FIG. 7, an implementation of another mass air chargeestimator model 700 will be described. The model 700 is a cylindervolume flow rate model that includes an intake manifold pressureestimator 705. The intake manifold pressure estimator 705 includes amanifold inflow estimator 710 and a manifold outflow estimator 730. Theinflow estimator 710 determines a throttle mass flow rate 711 based onthe throttle position 712, the pre-throttle air pressure 714 (which maybe the ambient air pressure or boosted pressure in a supercharged orturbo charged engine), the pre-throttle air temperature 715 (which maybe the ambient air temperature pressure or boosted temperature in asupercharged or turbo charged engine), and the current manifold pressure755. The throttle mass flow rate 711 is converted to a manifold intakemass flow rate 719 in terms of moles per unit time.

The manifold outflow estimator 730 uses engine crank angle 744 andknowledge of the engine dynamics, together with specific firingdecisions 732 and the estimated mass air charge of each fired cylinder775 to determine an estimated cylinder air usage (manifold out flow)741, which is also represented in terms of moles per unit time. Themanifold outflow estimator converts the per cylinder MAC 775 to a moleper unit volume value 724 using multiplier 722. Mole per unit volumevalue 724 is one input into multiplier 726. The other input intomultiplier 726 is cylinder displacement parameter 728, which representsthe volume of inducted gas per unit time. The cylinder displacementparameter 728 is based on a piston displacement model 733, whichdetermines piston position, and thus inducted gas volume, as a functionof crank angle 744. The cylinder displacement parameter is the output ofthe piston displacement model 733 when the firing decision 732 is afire, which may be represented as a “1” on signal line 732. If thefiring decision is a skip, which may be represented as a “0” on signalline 732, then multiplier 735 causes cylinder displacement parameter 728to remain zero.

The manifold intake flow 719 and the out flow 741 are combined by summer746 which provides a net manifold molar flow rate 747 (with the out flowbeing a negative number). Manifold model 750 scales the molar flow ratebased on the cylinder intake port temperature 751 using multiplier 752and converts that value to a pressure change rate which may beintegrated by integrator 753 to determine the estimated manifoldpressure 755. The estimated manifold pressure 755 is then used by MACestimator module 770 in the estimation of the mass air charge 775 usedin each specific induction event.

A variety of different estimation models may be used by MAC estimator770. In the illustrated embodiment, a speed density calculation is usedwhich utilizes engine speed 780, intake port temperature 751, intake camposition 734, exhaust pressure 782 and the estimated manifold pressure755.

It should be appreciated that the described approach compensates for theeffect of specific firing decisions by subtracting the mass of airextracted from the manifold in conjunction with each induction event.The mass air charges 775 associated with induction events areindividually calculated on an induction event by induction event basisso that the mass air charge calculation accounts for the variation inthe air charges associated with different induction events that tend tooccur as part of skip fire operation of an engine. These air chargeestimates, in turn, are used in the estimation of the manifold pressure755 which provides a better estimation of the manifold pressure atdifferent times, which may vary significantly between firingopportunities due to the effects of specific fire and skip (no inductionevent) decisions.

Referring next to FIG. 8, a variation of the previous embodiments isillustrated which provides an alternative embodiment suitable for usewith multi-level skip fire and/or dynamic multi-charge level engineoperation. The illustrated MAC estimator model 800 is quite similar tothe previously described embodiments except that it includes redundantmanifold outflow estimators 830(a) and 830(b) with each outflowestimator being associated with a different available firing level inthe current operational state. If more than two firing levels aresupported in a particular state, than an additional outflow estimator830 would be provided for each supported level. The manifold inflowestimator 810, the manifold model 850, and the MAC calculator 870 mayall be otherwise similar to the corresponding components described inany of the previous embodiments, with the differences being that the MACcalculator must account for the level of the induction event. Thus, theMAC calculator may take the cam state associated with each level as aseparate input with the firing decisions to determine which level to usein conjunction with each particular induction event.

The manifold outflow estimator architecture illustrated in FIG. 8 can beused to calculate the intake manifold outflow for both multi-level skipfire and multi-charge level engine operation in which all the cylindersare fired, but individual working cycles are purposely operated atdifferent cylinder output levels. Thus, it should be appreciated thatthe various estimators described are well suited for use in estimatingair charge any dynamic firing level modulation engine operation.

The MAC estimators described in FIGS. 4, 5, 7, and 8 all use a MAPvalue, either measured, predicted, or both, to assist in MACdetermination. In some embodiments, a predictive model of MAP behaviormay be applied iteratively on a cylinder event (firing opportunity byfiring opportunity) basis to provide an increasingly accurate estimationof MAC of a selected working cycle as the selected cycle draws closer toexecution. Such refinement of the MAC estimation has several advantages.In general, the MAC estimate may be used in a variety of differentcontrol schemes—however, the time that the estimate is needed may varysignificantly between the different applications. Iteratively refiningthe estimate allows the best possible estimate to be used at the timethat it is needed. For example, an early estimation of the MAC may beneeded for use in a torque model (e.g., the MAC estimation may be neededseveral crankshaft revolutions before the working cycle corresponding toa particular firing decision begins). In contrast, the fuel chargedetermination (which needs a precise air charge estimate for bestperformance) may not be made until close to the time of IVC in somedirect injection engines (i.e., after the corresponding working cyclehas begun). In this example, the fuel charge determination can utilize amore refined estimate of the MAC, which facilitates improved fueleconomy and improved emission characteristics. Of course, the MACestimate may be utilized by a variety other systems and controller, eachof which is able to work with the best possible estimate at the timethat the MAC estimate is needed.

FIG. 9 is a flow chart illustrating a representative method 900 ofproviding successively more accurate MAP and MAC estimates in askip-fire controlled engine according to an embodiment of the presentinvention. The method 900 may start or be triggered by a cylinder firingdecision at step 910. The illustrated algorithm is preferably repeatedeach cylinder event (firing opportunity) or more frequently.

Initially, the current value of the engine parameters that are relevantto the MAP and MAC estimation, such as current MAP, cam phase, throttleposition, exhuast gas recirculation valve position, etc., are obtainedor determined at step 920. The determination may be based on sensordata, actuator inputs, models, or various combinations thereof. Thesevalues may be arranged in a data array, which is associated with a timeor firing opportunity when the array elements were determined. In aparticular implementation, the initial values of the parameters P(0) mayinclude the present manifold pressure [MAP(0)], the present throttleposition [Throttle(0)], the presesnt firing decision [Fire(0)] and thepresent CAM position [CAM(0)] are identified at step 920.

Flow chart 900 then proceeds to step 930 where predictions are made forwhat the values of the relavant engine parameters will be for each of“n” subsequent firing opportunities, with “n” representing the number ofpotential intake events that will occur from the present time until theintake event associated with the last firing decision to be modeled(e.g., until the intake event associated with the latest firingdecision).

In some embodiments, the relavant engine parameters may include thepredicted throttle position at each iteration [Throttle(1:N)], thefiring decisions associated with each intervening firingopportunity/iteration [Fire(1:N)] (which is known), and the predictedcam position for each iteration [Cam(1:N)]. The present MAP is used asthe initial value MAP value [MAP(0)].

At step 935 a counter k, is initialized to zero. The logic then proceedsto step 940 where the value of the counter, k, is compared with n, thenumber of firing events between the firing decision, where flow chart900 was initiated, and execution of that decision. When the counter isless than n, the method proceeds to step 950. At step 950 the modelparameters associated with the current iteration (k) are obtained (e.g.,throttle(k), cam(k)) etc.) for use as inputs into the MAP model for thecurrent (k) iteration.

In step 960 a determination is made whether the current firingopportunity to be simulated is a skip or a fire. The appropriate model(fire or skip) is then used to predict the next MAP, which is saved butalso used to calculate a predicted MAC if the opportunity was a fire.More specifically, when the current firing opportunity is a fire, thelogic proceeds to step 970 where the fired cylinder MAP model isexecuted using the parameters for iteration (k) (using the parametersreferenced in step 950) to determine a predicted MAP for the nextiteration [MAP(k+1)]. The specific MAP model employed can vary widelybased on design goals and computational capabilities of the processorexecuting the MAP model. By way of example, the MAP model may utilizecontinuous time integration (as in some of the examples above), RungeKutta integration (which is less computationally intensive), or otherappropriate methods. After the MAP(k+1) is estimated, it is saved foruse in subsequent iterations.

If back in step 960 the current firing opportunity is a skip, then thelogic flow proceeds to step 980 where the skipped cylinder MAP model isexecuted to determine the MAP for the next iteration [MAP(k+1)].

After MAP(k+1) has been calculated, the corresponding MAC(k+1) can becalculated if firing opportunity (k+1) is a fire as represented by step975. Of course, the MAC associated with any skipped working cycle thatdoes not include an induction event will be zero as represented by step977. At this stage, both the predicted MAP and MAC for cylinder event(k+1) are known. This predicted MAP may then used in the calculation ofthe MAP and MAC for the next following cylinder event. Morespecifically, in the context of the flowchart 900, the value (k) isindexed in step 990 and the loop 940-990 is repeated until the MAP andMAC have been calculated for all of the cylinder events ofinterest—which is represented by (k) equaling (n) in step 940, whereinthe process ends.

As suggested above, the described process is repeated for each cylinderevent (firing opportunity). This has the effect of updating the MAP andMAC calculations for all of the cylinders of interest with the latestinformation every firing opportunity. These values can be used by theECU, skip fire controller and/or other controllers and processes intheir respective calculations whenever needed with the assurance thatthe values being used are the best MAC or MAP estimates available atthat time. Additionally, the ECU may use the estimated MAP or MAC data,in conjunction with other measurements, such as readings from an oxygensensor in the exhaust stream, to provide engine diagnostics and improvethe MAC and MAP models.

It should be appreciated that the order of steps and function of stepsmay be altered or reconfigured in many ways, while retaining modelcentral features of iteratively executing a MAP and MAC model to provideincreasingly accurate MAC estimates and using the known skip/firepattern as an input to or part of the model. While the loop in flowchart900 has been described as being executed for every firing opportunity insome embodiments, the loop 998 could be executed at regular time orcrank angle intervals with appropriate scaling adjustments to the loopcounter k appropriately. Flow chart 900 may also be readily modified fordynamic firing level modulation by adding different paths in the loop998 associated with different cylinder output levels or deleting thepath associated with a skipped firing opportunity.

Many of the embodiments described above refer to using intake andexhaust cam phases as inputs to the manifold outflow estimator. Camphase is an appropriate input for many outflow estimators because camphasers are commonly used to regulate the timing of the valve openingsand closings. However, it should be appreciated that other hardware canreadily be used to control the air charge inducted into any particularcylinder. For example, engines that have variable valve lifters canalter the air charge by varying the valve lift. In embodiments in whichvariable valve lifters are utilized, the position of the lifters willtypically be an input to both the manifold outflow estimator and the MACcalculator in addition to the cam phases (if cam phase control is alsoprovided) or instead of the cam phase (if separate cam phase control isnot provided). In other embodiments, electromechanical actuators (e.g.solenoids) may be used to operate the valves and in such embodiments,the relative timing of the opening and closing of the valves isimportant to the manifold outflow estimator and the MAC calculator andtherefore would be provided as inputs to those components. Moregenerally, it should be appreciated that whatever actuators are utilizedto actuate the intake and exhaust valves, the relevant parametersregarding the particular actuator(s) used may be used by the manifoldoutflow estimator and MAC calculator/estimator.

In the SAE paper referenced above, two types of firing history aredescribed. The first, is referred to as order history. The order historytracks what the firing sequence is for all of the cylinders. For anyparticular working cycle, the order based firing history relevant tothat working cycle will be indicative of what occurred in the sequenceof immediately preceding working cycles of all of the cylinderssequenced in their design firing order. That firing history can be thesequence of skips and fires for a skip fire controlled engine, or whenmulti-level skip fire or dynamic multi-charge level engine operation isused, it may additionally or alternatively include a history of therelative air charge or power output levels. The models described abovework well for accounting for order history induced variations in the aircharge.

The other type of firing history is the cylinder history. Each cylinderhas its own firing history—which is indicative what occurred in theprevious working cycle(s) of that particular cylinder. In general, acylinder that was skipped in its previous working cycle will be coolerthan a cylinder that was fired in its previous working cycle. Also, inan induction event following a skip, the exhaust and intake valves maynot be simultaneously open as is typically the case at the beginning ofa working cycle for a continuously firing cylinder. Both a lowercylinder temperature and the absence of valve overlap can impact thefresh air inducted into a cylinder following a skipped cycle. Theeffects of cylinder history can be incorporated into the air chargemodel as well. By way of example, in any of the described embodiments,the effects of cylinder firing history can be incorporated into the MACestimator (and thus the manifold outflow estimator), such as the speeddensity based MAC estimator 770 described with reference to FIG. 7. Insome implementations, the intake port temperature estimate used in theMAC estimator is based on the firing history to account for thetemperature effects that result from specific skip/fire decisions withinthat cylinder. In general, whether the cylinder was skipped or fired inthe immediately preceding working cycle in the cylinder has the greatestimpact on the MAC for a particular working cycle, although the impactsof a set of several previous skip/fire decisions corresponding to theimmediately preceding set of working cycles/firing opportunities canhave noticeable effects.

In some embodiments, the MAC estimator may be configured to make aninitial MAC estimate based on the assumption that the cylinder had beenfired each working cycle and a variable scaler may be provided to scalethe initial MAC estimate to a final MAC estimate based on the cylinderfiring history. In such an arrangement, a cylinder firing history basedlookup table can be used to determine the appropriate scaling factor foreach working cycle. In other embodiments a model may be used to predictcylinder temperature based on the particular cylinder's firing historyand any other factors deemed relevant.

The cylinder firing history may be tracked in a wide variety of manners.By way of example, some suitable approaches for tracking cylinder firinghistory are described in U.S. patent application Ser. No. 13/843,567,which is incorporated herein by reference.

The invention has been described in conjunction with specificembodiments, it will be understood that it is not intended to limit theinvention to the described embodiments. On the contrary, it is intendedto cover alternatives, modifications, and equivalents as may be includedwithin the spirit and scope of the invention as defined by the appendedclaims. The present invention may be practiced without some or all ofthese specific details. In addition, well known features may not havebeen described in detail to avoid unnecessarily obscuring the invention.For example, the concept of intake valve opening and closing has beendescribed in the context of a single intake valve; however, it is wellknown that a cylinder may use more than one intake valve and the closingtime of these valves may be different. In this case, the intake valveclosing time may refer to the closing time of a single valve or may besome type of average of the individual valve closing times. Also, thepresent invention has been described in terms of the induction eventoccurring at the beginning of a firing working cycle. This is not arequirement. In some embodiments of skip fire control air may beinducted into a cylinder and remain in the cylinder through multipleengine cycles before a spark ignites the air/fuel-mixture. Furthermore,the current invention has been described in terms of each skip having noinduction event. This is not a requirement. As described in U.S. Pat.No. 9,328,672 and U.S. patent application Ser. No. 15/009,533, bothassigned to Tula Technology Inc. and incorporated herein by reference,in some cases it may be desirable to have an induction event on askipped cylinder. In these cases, the method described in relation toFIG. 6 may be modified so that step 610 determines whether there is aninduction event, rather than a decision to fire. In this situation stepsrelated to determining an appropriate fuel mass and injecting that fuelmass may be omitted from the method.

Although the inventions have been described primarily in the context ofestimating air charge during skip fire operation of an engine, it shouldbe appreciated that the various estimators, methods and approachesdescribed are well suited for use in estimating air charge inassociation with any type of dynamic firing level modulation engineoperation.

In accordance with the present invention, the components, process steps,and/or data structures may be implemented using various types ofoperating systems, programming languages, computing platforms, computerprograms, and/or computing devices. In addition, those of ordinary skillin the art will recognize that devices such as hardwired devices, fieldprogrammable gate arrays (FPGAs), application specific integratedcircuits (ASICs), or the like, may also be used without departing fromthe scope and spirit of the inventive concepts disclosed herein. Thepresent invention may also be tangibly embodied as a set of computerinstructions stored on a computer readable medium, such as a memorydevice.

What is claimed is:
 1. A method of estimating an air charge during aselected working cycle during operation of an engine, the methodcomprising: determining that the selected working cycle will be fired,wherein the firing determination is made before an induction eventassociated with the selected working cycle begins; estimating a manifoldpressure at a time corresponding to the induction event associated withthe selected working cycle, wherein the manifold pressure estimateaccounts for impacts from one or more intervening potential inductionevents that will occur between the time that the manifold pressure isestimated and the time that the induction event associated with theselected working cycle occurs; and utilizing the estimated manifoldpressure in the estimation of the air charge for the selected workingcycle.
 2. A method as recited in claim 1, further comprising: utilizingthe estimated air charge in a determination of a desired fuel charge forthe selected working cycle; and causing the desired fuel charge to beinjected for the selected working cycle.
 3. A method as recited in claim1 wherein the air charge estimation is made while operating the engineis a skip fire operational mode.
 4. A method as recited in claim 1wherein the air charge estimation is made while operating the engine ina dynamic firing level modulation mode.
 5. A method as recited in claim1 wherein the air charge estimation utilizes a then current firinghistory of a cylinder in which the selected working cycle occurs in thedetermination the mass air charge.
 6. A method as recited in claim 1wherein for each intervening potential induction event that results inan actual intervening induction event, a then current firing history ofa working chamber corresponding to the actual intervening inductionevent is used in the manifold pressure estimation.
 7. A method asrecited in claim 1 further comprising predicting one or more intakevalve cam phases for use in the manifold pressure and air chargeestimations, wherein the predicted intake valve cam phase corresponds toa cam phase that is expected at the time of the induction eventassociated with the selected working cycle, and wherein the predictedintake valve cam phase is at least sometimes different than a currentintake valve cam phase at the time the air charge estimation is made. 8.A method as recited in claim 1 further comprising predicting one or moreexhaust valve cam phases for use in the manifold pressure and air chargeestimations, wherein the predicted exhaust valve cam phase correspondsto a cam phase that is expected at the time of the induction eventassociated with the selected working cycle, and wherein the predictedexhaust valve cam phase is at least sometimes different than a currentexhaust valve cam phase at the time the air charge estimation is made.9. A method as recited in claim 1 wherein the estimated manifoldpressure corresponds to a manifold pressure that is expected at the timeof closing an intake valve actuated to facilitate the induction eventassociated with the selected working cycle.
 10. A method as recited inclaim 1 wherein the air charge estimation is made in the range ofapproximately 1080° of crank angle rotation prior to the opening of anintake valve to closing of the intake valve actuated to facilitate theinduction event associated with the selected working cycle.
 11. A methodas recited in claim 10 wherein the air charge estimation is an initialair charge estimation, which is updated after execution of each of theintervening potential induction events prior to the opening of an intakevalve actuated to facilitate the induction event associated with theselected working cycle.
 12. A method as recited in claim 1 wherein theair charge estimation is further based in part on whether an exhaustvalve that vents the working chamber in which the selected working cycleoccurs was actuated in the immediately preceding working cycle in thatworking chamber.
 13. A method as recited in claim 12 wherein theestimated air charge is higher when the exhaust valve that vents theworking chamber in which the selected working cycle occurs was notactuated in the immediately preceding working cycle in that workingchamber relative to what the estimated air charge would have been if,all other factors being equal, the exhaust valve had been actuated inthe immediately preceding working cycle.
 14. A method as recited inclaim 1 wherein the engine is a piston engine having no more than fourcylinders and an intake manifold having a volume not greater than 10times the displacement of a cylinder.
 15. A method as recited in claim 1wherein the method is repeated for each fired working cycle while theengine is operated in a skip fire or other dynamic firing levelmodulation operational mode.
 16. A method as recited in claim 3 whereinthe manifold pressure estimation is made for each working cycle during aperiod of the skip fire operation and the air charge estimation is madefor each fired working cycle during the period of skip fire operation.17. A method as recited in claim 1 wherein the manifold pressure and aircharge estimation is updated after each of the one or more interveningpotential induction events until the selected working cycle is executed.18. A method as recited in claim 17 wherein each updated air chargeestimation provides a more accurate air charge estimation for theselected working cycle.
 19. A method of estimating cylinder air chargeon a cylinder intake event by cylinder intake event basis for an engineduring operation of an engine in a dynamic firing level modulation modein which firing level decisions are made on a working cycle by workingcycle basis, the method comprising: estimating the inflow of air into anintake manifold; estimating an outflow of air from the intake manifold,wherein the air outflow estimate includes, for each of a multiplicity ofspecific potential cylinder intake events, estimating an amount of airwithdrawn from the intake manifold corresponding to that specificpotential cylinder intake event, wherein each specific potentialcylinder intake event corresponds to an associated working cycle havinga corresponding firing level decision and wherein the air outflowestimation utilizes specific firing level decisions or specific cylinderintake decisions to help determine the amount of air withdrawn from theintake manifold during respective corresponding specific potentialcylinder intake events; estimating a manifold pressure based at least inpart on the estimated air inflow and estimated air outflow; andutilizing the estimated manifold pressure in individual estimation of anair charge for each cylinder intake event in a series of cylinder intakeevents, wherein the estimated manifold pressure is updated for eachindividual air charge estimation, whereby each of the individual aircharge estimations factor the impact of at least one potential cylinderintake event that has not yet occurred but for which the associatedfiring level decision has been made at the time such individual aircharge estimation is made.
 20. A method as recited in claim 19 whereinthe air charge estimates are made while operating the engine is a skipfire operational mode.
 21. A method as recited in claim 19 wherein theair charge estimates are made while operating the engine in amulti-charge level engine operational mode in which while operating in aparticular state, all cylinders are fired but different working cyclesare fired at different output levels in an interspersed manner.
 22. Amethod as recited in claim 19 wherein the estimated air charge isdetermined using a speed density calculation.
 23. A method as recited inclaim 19 wherein for each cylinder intake event, the mass air chargeestimation utilizes a then current firing history of the correspondingcylinder in the determination the mass air charge.
 24. A method asrecited in claim 19 wherein the estimation of the outflow of the airfrom the intake manifold utilizes a then current firing history of thecorresponding cylinder in the determination of the amount of airwithdrawn from the intake manifold for each cylinder intake eventaccounted for.
 25. A method as recited in claim 19, further comprising,for each of at least some of the cylinder intake events: directly orindirectly utilizing the corresponding estimated air charge in adetermination of an amount of fuel to provide for the correspondingworking cycle; and injecting the determined amount of fuel for use inthe corresponding working cycle.
 26. An engine controller for aninternal combustion engine having a plurality of working chambers, eachworking chamber being configured to operate in a series of workingcycles, each working cycle having a potential induction event, theengine controller comprising: a dynamic firing level modulationcontroller arranged to determine a firing level for each working cycle;a manifold pressure estimator arranged to estimate a manifold pressurefor a selected one of the working cycles, and arranged to make themanifold pressure estimation (i) before an induction event associatedwith the selected working cycle begins, and (ii) after the firing leveldetermination is made for the selected working cycle, the manifoldpressure estimation being arranged to account for impacts from one ormore intervening potential induction events that will occur between thetime that the manifold pressure estimate is made and the time that theinduction event associated with the selected working cycle occurs, eachpotential induction event being associated with a differentcorresponding working cycle; and an air charge estimator configured toestimate an air charge for the selected working cycle based at least inpart of the estimated manifold pressure for the selected working cycle.27. An engine controller as recited in claim 26 wherein the dynamicfiring level modulation controller is a skip fire controller arranged todirect operation of the engine in a skip fire operational mode.
 28. Anengine controller as recited in claim 26 wherein the dynamic firinglevel modulation controller is one of: a) a dynamic multi-level firingcontroller arranged to direct operation of the engine in a multi-levelfiring operational mode in which working cycles having at least twodifferent effective displacement levels are interspersed while operatingat a given state; and b) a dynamic multi-level skip fire firingcontroller arranged to direct operation of the engine in a multi-levelfiring operational mode in which working cycles having at least twodifferent effective displacement levels and skipped working cycles areall interspersed while operating at a given state.
 29. An enginecontroller as recited in claim 26 wherein the air charge estimatorutilizes a then-current firing history of a working chamber in which theselected working cycle occurs in the determination of the mass aircharge.
 30. An engine controller as recited in claim 26 wherein for eachintervening potential induction event that corresponds to an actualinduction event, the manifold pressure estimator is configured to use athen-current firing history of a working chamber corresponding to theactual intervening induction event in the manifold pressure estimation.31. An engine controller as recited in claim 26 further comprising anintake valve cam phase predictor configured to predict an expectedintake valve cam phase, corresponding to an intake valve cam phase thatis expected at the time of the induction event associated with theselected working cycle for use by the manifold pressure estimator andthe air charge estimator.
 32. An engine controller as recited in claim26 further comprising an exhaust valve cam phase predictor configured topredict an expected exhaust valve cam phase, corresponding to an exhaustvalve cam phase that is expected at the time of the induction eventassociated with the selected working cycle for use by the manifoldpressure estimator and the air charge estimator.
 33. An enginecontroller as recited in claim 26 wherein the manifold pressureestimator is arranged to estimate a manifold pressure at a time that anintake valve actuated to facilitate the induction event associated withthe selected working cycle is closed.
 34. An engine controller asrecited in claim 26 wherein the air charge estimator is arranged toestimate the air charge in the range of 180° to 1080° of crank anglerotation prior to the opening of an intake valve actuated to facilitatethe induction event associated with the selected working cycle.
 35. Amethod as recited in claim 34 wherein the air charge estimation is aninitial air charge estimation, which is updated after execution of eachof the intervening potential induction events prior to the closing of anintake valve actuated to facilitate the induction event associated withthe selected working cycle.
 36. An engine controller as recited in claim26 further comprising a fuel charge estimator, wherein the fuel chargeestimator is configured to utilize the estimated air charge in adetermination of a desired fuel charge for the selected working cycle,and wherein the engine controller is further configured to cause thedesired fuel charge to be injected for the selected working cycle. 37.An engine controller as recited in claim 26 wherein the air chargeestimation is further based in part on whether an exhaust valve thatvents the working chamber in which the selected working cycle occurs wasactuated in the immediately preceding working cycle in that workingchamber.
 38. An engine comprising: an engine controller as recited inclaim 26; an intake manifold; and a plurality of cylinders that receiveair from the intake manifold, wherein the internal volume of the intakemanifold is no greater than 10 times the displacement of a cylinder. 39.An engine as recited in claim 38 wherein the engine is a four cylinderpiston engine.
 40. A method of estimating air charge during a selectedworking cycle during skip-fire operation of an engine, the methodcomprising: making a skip or fire decision regarding all interveningfiring opportunities that occur between the selected working cycle andthe time of making the air charge estimate; and updating the air chargeestimate after execution of each intervening firing opportunity, whereinthe updated air charge estimate is made using engine parametersassociated with the most recently executed firing opportunity.
 41. Amethod as recited in claim 40 wherein the engine is operating in adynamic firing level modulation mode with fired cylinders havingdifferent cylinder outputs.
 42. A method as recited in claim 40 whereinthe estimated air charge is determined using a speed densitycalculation.
 43. A method as recited in claim 40 wherein the estimatedair charge used to help determine injected fuel mass is the estimatedair charge determined immediately prior to programming fuel injectorsthat deliver the fuel mass.
 44. A method as recited in claim 40 whereinthe updated air charge estimate is based on a manifold inflow andmanifold out flow model.